Mercaptans are of great industrial interest and are very widely used nowadays by the chemical industries, particularly as precursors or starting materials for the synthesis of more complex organic molecules. For example, methyl mercaptan (CH3SH) is used as starting material in the synthesis of methionine, an essential amino acid which is used to a high degree in animal feeding. Methyl mercaptan is also used for the synthesis of dialkyl disulphides, and particularly for the synthesis of dimethyl disulphide (DMDS), an additive for sulphurizing hydrotreating catalyst for petroleum fractions, among other applications.
Methyl mercaptan is nowadays commonly produced industrially on the tonne scale from methanol (CH3OH) and hydrogen sulphide (H2S) in accordance with the following reaction (1):CH3OH+H2S→CH3SH+H2O  (1)
This synthesis pathway, however, has a number of drawbacks, including that of using methanol, which necessitates a supplementary step, since methanol is prepared from hydrocarbon charges, and the drawback of leading to secondary products, typified especially by dimethyl ether (CH3OCH3), dimethyl sulphide (CH3SCH3), and cracking products (such as, for example, carbon monoxide and carbon dioxide), and water, to state only some of the drawbacks. Moreover, the presence of secondary products of these kinds results in a large number of purification steps for the methyl mercaptan, to the detriment of high productivity and high selectivity and therefore of an optimum yield.
This synthesis pathway, and also some improvements thereto, are described for example in documents WO2004/096760, WO2006/015668, WO2007/028708, WO2008/118925 and WO2013/092129.
Other synthesis processes do away with the need to use methanol, and include the preparation of methyl mercaptan from carbon monoxide (CO) in accordance with the following reaction (2):CO+2H2+H2S→CH3SH+H2O  (2)
However, the use of carbon monoxide (CO) is not free of drawbacks, since CO originates essentially from synthesis gas, which is a CO/H2 mixture, and which consequently necessitates:
a supplementary step of steam reforming of a hydrocarbon source for the purpose of obtaining a synthesis gas,
the availability of a synthesis gas having appropriate proportions of carbon monoxide (CO) and hydrogen (H2), without any need for adjustment to the CO/H2 ratio by means of the water-gas shift reaction, defined as follows in reaction (A):CO+H2O→CO2+H2  (A)
Moreover, the processes in accordance with reaction (2) above have the drawback of giving rise to secondary products, such as carbon dioxide (CO2), methane (CH4), dimethyl sulphide (CH3SCH3) and water (H2O). These processes are described for example in documents US2007213564, US2008293974, US2010094059, and US2010286448.
Yet other processes are described in the literature, and combine different reactions, such as:
formation of CS2 and H2S from methane and sulphur, in accordance with reaction (3):CH4+4S→CS2+2H2S  (3)
hydrogenation of CS2, using the hydrogen formed above, in accordance with reaction (4):CS2+3H2→CH3SH+H2S  (4)
These processes evidently combine the drawbacks described for reactions (1) and (2) with the additional difficulty of necessitating a supplementary source of hydrogen in order to perform reaction (4).
Yet another method is disclosed in document WO2010/046607 and involves the hydrogenation of sulphur compounds which carry C═S unsaturation, and more particularly the hydrogenation of carbon disulphide (CS2), to methyl mercaptan (CH3SH) in accordance with reaction (4) above.
However, the process performed in this document employs carbon disulphide (CS2), which is a dangerous, toxic product which can be used industrially with the installation of severe safety means; all enterprises and factories do not wish to or are unable to develop a plant meeting the safety standards required for the holding of carbon disulphide.
International patent application WO2001/96290 proposes a process for synthesis of methyl mercaptan directly from methane (CH4) and H2S with co-production of hydrogen. This direct reaction between methane and H2S is accomplished by means of a pulsed plasma with corona discharge. This patent application does not describe any example of synthesis, and it does not appear possible to envisage the industrial-scale implementation of this process for synthesizing methyl mercaptan. Moreover, this process requires the synthesis of H2S if the latter is unavailable.
Today, therefore, there is a need for a process for synthesis of methyl mercaptan that does not exhibit the drawbacks encountered in the known processes, thus being more environmentally friendly and less eco-toxic, but also safer, while preserving the high yields and selectivities, or even with yields and selectivities which are improved relative to those of the known processes, while being a process which is operated as economically as possible.